Why we use a Base Matching Unit (BMU) with End/Base-Fed Verticals and Inverted-Ls

In this article we look at the reduction in feedline loss when we use a BMU to mitigate impedance extremes at the base feedpoint of a Vertical or Inverted-L antenna. Naturally, these antennas are worked against a proper counterpoise radial system for decent efficiency, especially on the low bands.

We look at the cases of a 42 ft antenna, and use an average of the measured and modeled feedpoint impedance to calculate feedline losses* for 50 ft of RG8x and for 100 ft LMR400.

The BMU referred to here is:

A 9:1 UNUN for the mid and high bandsA Loading Coil, switched in instead of the UNUN, for the low bands

Note that feedline losses are shown for a 42 ft EF-Inverted-L or Vertical. This represents a worst case configuration. Feedline losses are a bit lower for longer EF-Inverted-Ls and Verticals, such as the typical 43 ft vertical and 45 ft Inverted-Ls.

BMU vs Direct Feed – Cable loss with 50 ft RG8x feedline. Losses are greatly reduced throughout the frequency range using a BMU with a switched-in Loading Coil on the lowest band ===>

BMU vs 9:1 Unun – Cable loss with 50 ft RG8x feedline. Losses are greatly reduced at the lower end of the frequency range using a BMU with a switched-in Loading Coil on the lowest band, as opposed to a simple Unun ===>

BMU vs Direct Feed – Cable loss with 100 ft LMR400 feedline. Losses are greatly reduced throughout the frequency range using a BMU with a switched-in Loading Coil on the lowest band ===>

BMU vs 9:1 Unun – Cable loss with 100 ft LMR400 feedline. Losses are greatly reduced at the lower end of the frequency range using a BMU with a switched-in Loading Coil on the lowest band, as opposed to a simple Unun ===>